The present disclosure relates to an anode for a lithium ion secondary battery that contains an anode active material containing silicon (Si) as an element, a lithium ion secondary battery including the same, an electric power tool using the lithium ion secondary battery, an electrical vehicle using the lithium ion secondary battery, and an electric power storage system using the lithium ion secondary battery.
In recent years, portable electronic devices such as video cameras, digital still cameras, mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a small and light-weight secondary battery capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (lithium ion secondary battery) is extremely prospective, since such a secondary battery is able to provide a higher energy density compared to a lead battery and a nickel cadmium battery.
The lithium ion secondary battery includes a cathode, an anode, and an electrolytic solution. The anode has an anode active material layer provided on an anode current collector. The anode active material layer contains an anode active material that is involved in the charge and discharge reaction.
A the anode active material, a carbon material has been widely used. However, in recent years, as further improvement in battery capacity is demanded, the use of silicon is being considered. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is expected that the battery capacity is thereby highly improved. In this case, the anode active material is not limited to a simple substance of silicon, and alloys and compounds thereof are also being considered.
However, in the case where silicon is used as the anode active material, while the battery capacity improves, some disadvantages occur. Specifically, since the anode active material intensely expands and shrinks during charge and discharge, the anode active material layer is easily pulverized. Further, since reactivity of the anode active material is high, decomposition of the electrolytic solution easily occurs.
Therefore, various studies are being conducted to improve various performances of the lithium ion secondary battery using silicon as the anode active material.
To improve charge and discharge characteristics, metal particles (particle size=0.0005 μm to 10 μm, both inclusive) are provided on a surface of an electrode active material in the cathode or the anode (for example, see Japanese Unexamined Patent Application Publication No. Hei 11-250896). To suppress increase in resistance within the battery and decrease in capacity, a second active material layer that contains metal and the like that forms an alloy with the lithium ion, and metal and the like that does not form an alloy with the lithium ion, is provided on a first active material layer that inserts and extracts lithium ions (for example, see Japanese Unexamined Patent Application Publication No. 2003-217574). To improve charge and discharge cycle characteristics, metal is contained on the surface of a thin film of which the main constituent is silicon (for example, see Japanese Unexamined Patent Application Publication No. 2003-007295). To improve cycle life, a surface coating layer composed of a conductive material having low lithium-compound forming ability is provided on an active material layer composed of a silicon material (for example, see Japanese Unexamined Patent Application Publication No. 2004-228059). To improve charge and discharge cycle characteristics, the surfaces of silicon-containing particles (average particle size (D50)=0.1 μm to 10 μm, both inclusive) are coated with a metal thin film (for example, see Japanese Unexamined Patent Application Publication No. 2005-063767). To obtain excellent charge and discharge efficiency, an anode material is used in which the surface of a reaction portion containing silicon is provided with a coated portion composed of a metal oxide (for example, see Japanese Unexamined Patent Application Publication No. 2007-141666). To improve electron conductivity, the anode active material layer contains a ferromagnetic metal (for example, see Japanese Unexamined Patent Application Publication No. 2007-257866). In this case, the anode active material layer has magnetization, and a maximum magnetization intensity obtained by a magnetization curve is 0.0006 T (tesla) or more. To reduce stress concentration and improve characteristics, a metal element is contained within the anode active material layer such that concentration increases and then decreases in the thickness direction (for example, see Japanese Unexamined Patent Application Publication No. 2007-257868). To reduce excess voltage at initial charging, at least a portion of the surfaces of the particles of the active material is coated with a metal material having low lithium-compound forming ability (for example, see Japanese Unexamined Patent Application Publication Nos. 2008-016195, 2008-016196, 2008-016198, 2008-066278, and 2008-277156).